1. Field of the Invention
This invention relates to a stencil mask for use with a photochemical reaction process, and a method of making such a stencil mask, and more particularly to a stencil mask for selectively etching and depositing an object to be processed at particular surface areas thereof by applying a reactive gas and light beams thereto.
2. Description of the Prior Art
At present, a plasma-assisted reactive-ion etching (RIE) method has been in wide use to manufacture semiconductor devices. However, the more the semiconductor devices are integrated and miniaturized, the more extensively the operation of components thereon is affected by damage caused during the manufacturing process. In other words, the reactive-ion etching method is prone to a problem that it tends to damage semiconductor devices extensively during the etching process, which leads to reduced yields of semiconductor devices.
To overcome this problem, it is necessary to set low ion energy in the reactive-ion etching process. As reported in "Semiconductor News", page 31, Oct., 1988, reduction in the ion energy lowers the etching anisotropy, which makes it to difficult to form minute precise patterns.
The use of the photochemical reaction process, referred to in HYOMEN KAGAKU, Vol. 5, No.4, page 435, has drawn our attention with its application to the manufacture of semiconductors. The photochemical reaction process uses both a reactive gas (e.g., a gas containing halogen) and photons. Specifically, photons are irradiated onto certain areas of the object, e.g., a surface of a semiconductor substrate, and the reactive gas is applied to the irradiated areas so as to etch them selectively. A fluoride gas such as sulfur hexafluoride (SF.sub.6) is used as the reactive gas. Photons enhance the etching process. In other words, since charged particles which are used in the reactive-ion etching are absent, the surface of the substrate will not be damaged.
Further, no intimate contact type photoresist mask is necessary in the photochemical reaction process although such a mask is indispensable in photoresist lithography. This process is advantageous since it can dispense with a photoresist mask forming process.
FIG. 25 of the accompanying drawings shows a cross-sectional view of a stencil mask for the photochemical reaction process. The stencil mask comprises a mask substrate 20, a window 21, a self-supportive film 22, a pattern 23, a rear reinforcement film 24, an intermediate film 25, and an absorber film 26.
The mask substrate 20 is made of silicon (Si) which has been in wide use in semiconductor technology due to its reliability.
The window 21 is formed in the mask substrate 20, and photons and reactive gas pass through the pattern 23 to the surface of an object to be processed.
The support film 22 is formed on the mask substrate 20 where the mask substrate 20 is in close contact with the object. The support film 22 enhances tension (internal stress) applied to the absorber film 26, thereby preventing the absorber film from being peeled off or loosened. The support film 22 is usually made of boron nitride (BN).
The rear reinforcement film 24 is formed on the rear surface of the mask substrate 20 so as to offset the internal stress generated by the support film 22, thereby keeping the mask substrate 20 from being bent. The rear reinforcement film 24 is made of boron nitride (BN) similarly to the support film 22.
The intermediate film 25 covers the surfaces of the support film 22 and the rear reinforcement film 24, and an exposed surface of the mask substrate 20. The intermediate film 25 enhances the adhering force between the support film 22 and the absorber film 26, and the adhering force between the rear reinforcement film 24 and the absorber film 26. The intermediate film 25 is made of chromium (Cr), for example.
The absorber film 26 covers the entire surface of the intermediate film 25, and absorbs or reflects photons which are incident outside the surface of the object. The absorber film 26 is made of gold (Au).
The pattern 23 includes a mask pattern made of the support film 22, and is covered by the intermediate film 25 and the absorber film 26. The pattern 23 is positioned on the front surface of the mask substrate 20 at a region corresponding to the window 21.
FIGS. 26 to FIG. 28 show processes for making the stencil mask for the photochemical reaction process of the prior art.
In a first step, the mask substrate 20 is covered with the self-supportive film 22 on the front surface thereof, and with the rear reinforcement film 24 on the rear surface thereof. Referring to FIG. 26, a part of the rear reinforcement film 24 is cut so as to form a part of the window 21.
In a second step, a through-hole is made by back-etching in the mask substrate 20 from the rear surface thereof, serving as the window 21, as shown in FIG. 27.
In a third step shown in FIG. 28, the pattern 23 is prepared on the self-supportive film 22 at the window region. Specifically, the mask pattern is depicted by an ion-milling process according to the focussed ion beam (FIB) technique.
In a fourth step, as shown in FIG. 25, the intermediate film 25 and the absorber film 26 are sequentially formed over all of the surfaces of the self-supportive film 22 and the rear reinforcement film 24 including the basic pattern.
The foregoing stencil mask is prone to the following problems.
First of all, the self-supportive film 22 is used so as to enhance the stress applied to the absorber film 26. In other words, the mask substrate 20 is covered with three films, i.e. the self-supportive film 22, the intermediate film 25, and the absorber film 26, which complicates the stencil mask structure. This complicated structure requires very complicated calculation of an amount of light transmission and diffraction, which would make it difficult to transfer minute precise patterns onto the object.
Secondly, the pattern 23 includes the basic pattern on the self-supportive film 22, and the intermediate film 25 and the absorber film 26 covering the basic pattern. Therefore, even if the basic pattern on the self-supportive film 22 is very precise, the presence of the intermediate film 25 and the absorber film 26 adversely affect the quality of the pattern 23.
Thirdly, the three films are formed on the mask substrate 20 as described above, which means an increase in the mask making process.
Finally, the basic pattern is directly depicted on the self-supportive film 22 by the focussed ion beam (FIB) process. Further, a great dose of focussed ion beams should be applied to depict the basic pattern on the absorber film 26. Scanning is also necessary in the focussed ion beam process, which means that it takes time to depict the basic pattern, i.e., a lot of time is necessary to manufacture the stencil mask.